Fabrication of narrow-band thin-film optical filters

ABSTRACT

A multi-layer thin-film narrow-band filter is fabricated by a process wherein each half of a symmetric stack is deposited under exact conditions, either successively or contemporaneously. The two halves of the filter structure are then combined and bonded together using a wet bonding process that remains reversible for a period of time sufficient for testing the filter and, if necessary, for separating the two halves to regain access to the spacer layer. Accordingly, the spacer layer may be adjusted to shift the peak wavelength, if necessary, to the precise design specifications for a particular application. After correction, the components of the symmetric filter are bonded again using the same wet process.

RELATED APPLICATIONS

This application is a continuation-in-part application of Ser. No.11/056,741, filed Feb. 11, 2005, issued as U.S. Pat. No. 7,332,044 onFeb. 19, 2008, which was based on U.S. Provisional Application Ser. No.60/544,447, filed Feb. 13, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the general field of optical filters and, inparticular, to the manufacture of multi-layered thin-film opticalstructures.

2. Description of the Prior Art

Optical filters are widely used as components of optical systems. Inparticular, they are employed in various spectroscopic and opticalcommunication systems where information is transmitted within verynarrow spectral bands. In order to retrieve the information contained ina particular band, the remaining spectral background (or spectral noise)has to be filtered out. This is achieved generally using optical filtersthat pass or reject (stop) the input light within a predeterminednarrow-band spectral range.

A wide variety of optical filters is known in the art, the mostfundamental one being a simple filter fabricated by depositing athin-film stack of appropriately selected optical material on a suitablesubstrate. As commonly understood in the art, the terms “thin” and“optically thin” refer to an optical thickness less than or on the orderof the wavelength of the light interest. For example, athird-of-a-micron thick layer of titania (Ti₂O₅) with a refractive indexof about 2.1 in the visible portion of the spectrum is consideredoptically thin for visible light.

The spectral characteristics of multi-layer thin-film filters aredetermined by the combination of properties of the layered materials(such as refractive index, reflectance, transmittance, absorbance,density, and homogeneity) and their physical configuration (suchthickness and the order of the layers in a stack). Narrow-bandinterference filters typically require symmetrical structures for properspectral performance. That is, each layer in the structure has acorresponding layer symmetrically disposed within the multi-layer stack.Multiple such stacks may also be combined to form more complex filters.

Conventional techniques for fabricating thin-film optical filtersinclude, for example, vacuum vapor deposition, deposition byelectron-beam evaporation (EBE), techniques based on ion-assisteddeposition (IAD), reactive ion plating, and ion-beam sputtering. Alltechniques utilize the same well-established manufacturing sequence. Asillustrated in the multi-layer structure 10 of FIG. 1, a thin-film stack12, sometime consisting of hundreds of layers, is deposited on asubstrate 14 made of suitable optical material. The deposition iscarried out in a vacuum chamber under well defined and controlledfabrication conditions and in a strict sequential order designed toproduce specific filter characteristic, starting with the first layer 1and sequentially building the stack up to the last layer n.

As is well understood in the art, narrow-band multi-layer filtersinclude a symmetrical structure wherein a spacer layer separates twomirror-image multi-layered components. Thus, as illustrated in itssimplest form in FIG. 2, the stack 12 includes a spacer layer 16 that isalso formed during the sequential deposition process. All other layersare deposited such that each layer in the bottom half-stack 18 has acorresponding layer symmetrically deposited in the top half-stack 20.That is, layer 1 is intended to be exactly the same as layer n, layer 2the same as layer n−1, and so on. A second optical substrate 22 may bealso deposited or laminated on top of the multi-layer stack 12 toprotect and increase the rigidity of the filter.

The sequential layer deposition of optical material is characterized byan inherent worsening of the material micro-structure in the layers(surface roughness, density, and presence of columnar structure withinthe volume) as the deposition progresses. This progressive deteriorationis due in part to the material and the surface quality of the substrate14 and in part to the conditions of deposition. It is known thatdeposition of thin films with very smooth surfaces requires that anextremely smooth and polished substrate be used (with a residualroot-mean-square roughness of a few Angstrom, at least less than ananometer and preferably about 1 Å for ceramics and metals). However,even under such ideal conditions, the residual structural defects andmicroscopic non-uniformities of each underlying layer propagate throughthe thickness of each newly deposited layer and grow more pronounced inthe upper layers of the stack. This shortcoming is an especiallycritical problem in the fabrication of narrow-band interference filtersbecause it materially affects the structural symmetry of the filter(even though macroscopic symmetry may be present). Since lightscattering due to structural non-uniformities inevitably leads tobroadening of the filter's band, in practice this shortcoming hasprevented the reliable fabrication of extremely narrow-band filters(i.e., filters with bandwidths on the order of a few Angstrom or less),especially notch filters.

As a result of such structural deficiencies, the layers of each pair ofsuch symmetrically disposed layers in the stack 12 are not structurallyidentical. Therefore, they do not perform optically in the same wayunder equal ambient conditions, which worsens the performance of thefilter as a whole by broadening its bandwidth and shifting the peakwavelength of the band. This effect is further worsened by the fact thatcorresponding layers in the stack, because of their microscopicdifferences, also tend to react differently to ambient stresses, such astemperature and humidity changes after the stack is removed from thefabrication chamber.

Still another problem lies in the fact that, once a conventional filterhas been fabricated, it is practically impossible to correct itsspectral performance (such as its precise peak wavelength) by accessingand modifying the inner spacer layer 16 of the filter. This deficiencyis very important, especially for very narrow-band etalon-type thin-filmfilters where the cavity provided by the spacer layer 16 determines thespecific spectral characteristics of the filter. An error in thethickness of the spacer layer leads to a spectral shift of the peakwavelength of the interference filter. Therefore, the filter cannot beused for the intended purpose and is practically wasted.

These process drawbacks of the prior-art are unavoidable and contributeto the current very high cost of manufacture of narrow-band multi-layerthin-film filters. Therefore, there remains a need for a manufacturingapproach that overcomes the problems and limitations described above.

SUMMARY OF THE INVENTION

This invention provides a new approach to the manufacture of multi-layerthin-film filters which allows the fabrication of filters withsignificantly reduced spectral bandwidth and permits post-fabricationtuning, if necessary, to the exact spectral-position specification ofthe filter's band. The invention is based on the exploitation of twodistinct ideas in the process of multi-layer filter manufacture.

According to one aspect of the invention, the fabrication sequence iscarried out in a way that reduces the number of thin-film layers thatare grown consecutively on top of one another in a filter stack. Thisapproach significantly reduces the degree of structural non-uniformitiespropagated within the layers of the stack during the film depositionprocess and maximizes the structural symmetry of the filter. This isachieved by employing the process in which each half of the symmetricstack is deposited under exact conditions, either successively orcontemporaneously.

The two halves of the filter structure are then combined and bondedtogether at the interface between the thin films constituting the toplayers of the two halves, typically corresponding to a spacer layer inthe symmetrical structure, to produce the complete symmetric stack ofthe filter. According to another aspect of the invention, this step iscarried out using a wet bonding process that has been found to producean interface of optical quality as good as found at the interfacebetween deposited films. Moreover, the bonding process remainsreversible for a period of time sufficient for testing the filter and,if necessary, for separating the two halves to regain access to thespacer layer. Accordingly, the spacer layer may be adjusted to shift thepeak wavelength of the filter's band as needed to meet the designspecifications for a particular application. After such correction, thecomponents of the symmetric filter are bonded again using the same wetprocess.

Various other aspects and advantages of the invention will become clearfrom the description in the specification that follows and from thenovel features particularly pointed out in the appended claims.Therefore, to the accomplishment of the objectives described above, thisinvention consists of the features hereinafter illustrated in thedrawings, fully described in the detailed description of the preferredembodiments, and particularly pointed out in the claims. However, suchdrawings and descriptions disclose only some of the various ways inwhich the invention may be practiced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically the order of fabrication and thestructure of a conventional multi-layer thin-film filter.

FIG. 2 illustrates schematically the structure of a conventionalsymmetric thin-film including a spacer layer sandwiched between twosubstantially equal multi-layer stacks in mirror-image disposition.

FIG. 3 is a schematic representation of a multi-layer thin-filmhalf-stack, including half of the spacer layer, deposited according tothe present invention as a step of the fabrication of a narrow-bandthin-film filter.

FIG. 4 is a schematic representation of an exact replica of themulti-layer thin-film half-stack, including the other half of the spacepayer, deposited according to the present invention as another step inthe fabrication of a narrow-band thin-film filter.

FIG. 5 is a schematic representation of the thin-film filter produced bywet bonding of the half-stacks of FIGS. 3 and 4.

FIG. 6 illustrates the shift in the vicinity of the design peakwavelength in the transmission spectrum of a flawed filter and thepost-fabrication correction produced by the present invention.

FIG. 7 illustrates schematically another multi-layer thin-filmhalf-stack, including the entire spacer layer, of a narrow-band filterfabricated according to the invention.

FIG. 8 is the same half-stack of FIG. 7, but without any spacermaterial, deposited according to the invention.

FIG. 9 is the narrow-band filter resulting by the combination of thehalf-stacks of FIGS. 7 and 8.

FIG. 10 illustrates a generic thin-film filter manufactured according tothe invention, wherein multiple symmetric stacks are fabricatedseparately and combined to form the structure of the filter.

FIG. 11 illustrates another generic thin-film filter according to theinvention, wherein only portions of the filter are symmetric withrespect to an arbitrary plane in the filter's structure.

FIG. 12 is a three-dimensional view showing the narrow-band thin-filmfilter of the invention integrated with generic optical components.

DETAILED DESCRIPTION OF THE INVENTION

The heart of the invention lies in the idea of substituting theconventional sequence of fabrication of symmetric thin-film filters(which always progresses sequentially by depositing each layer on top ofthe previous one, starting with the first layer in the stack and endingwith its last layer) with a process wherein only each half of thesymmetric stack is deposited sequentially. The two half stacks are thencombined to produce the symmetric full-stack structure of the filter.

Narrow-band filters fabricated with this two-step approach producesignificant advantages over analogous filters manufactured byconventional methods. First, they exhibit a substantially narrower widthin the stop (or pass) band of interest, which is critically important inmany applications. Second, the filter can be adjusted after fabricationto correct the exact spectral position of the filter's stop (or pass)band, if necessary, which is not possible with conventional filters.

According to one aspect of the invention, an advantage is achieved byfabricating each matching half-stack of the symmetric filter stack underconditions that produce substantially the same surface imperfections ineach pair of corresponding layers in the two half-stacks, rather thandepositing the entire stack in a single setting during which the surfaceimperfections of each underlying layer vary progressively as a result ofpropagation and produce different physical characteristics incorresponding layers of the two half-stacks. This approach enables thefabrication of multi-layer filter stacks with maximum structuralsymmetry. Therefore, the functional equivalence of the layers isoptimized, producing a narrower spectral band than otherwise achievableby multi-layer deposition, as described in greater detail below.

According to another aspect of the invention, the two equal half-stacksso produced are combined to form the symmetrical structure of the filterusing a wet bonding process which remains reversible for a period oftime sufficient to test the characteristics of the filter. Therefore, ifthe desired specifications are not met, the filter can be taken apart toadjust the thickness of the spacer layer as needed for a particularapplication. Those skilled in the art will recognize that bothadvantages are exceptionally important in the construction ofinterference filters, especially in the case of extremely narrow-bandinterference filters (with bands on the order of several Angstrom orless) that comprise hundreds of layers.

Referring to the figures, wherein like parts are designated throughoutwith like numerals and symbols, FIGS. 3-5 illustrate schematically thestructural components of thin-film stacks obtained at various stages offabrication according to the method of the invention. FIGS. 3 and 4 showtwo matching elements 30 and 40 fabricated in a first stage, eithersimultaneously or sequentially, under identical manufacturing conditions(that is, using the same sequence of materials and the same depositionconditions and parameters). Using conventional methods of deposition,each of the multi-layer thin-film half-stacks 32,42 of the matchingelements 30,40 is deposited on respective substrates 34,44, which may ormay not be the same. Each substrate is preferably polished to highquality so that the residual root-mean-square (rms) roughness of theirsurfaces does not exceed a few Angstrom (in any case less than onenanometer and, in the case of metals and ceramics, preferably about 1Å). This requirement of substrate preparation is discussed in furtherdetail below. The deposition of the half-stacks 32,42 may be followed,if desired, by the conventional deposition of respective spacer layers36,46 on top of the last layers m and m of each half-stack (note thatboth stacks necessarily have the same number of filter layers, m).

As a result of the same deposition process used for each matchingelement 30 and 40, two matching multi-layer structures are formed withterminating upper surfaces 38,48. Because the half-stacks 32,42 aredeposited over respective substrates in conventional sequential fashionunder the same deposition conditions (starting with the lowest layer 1and ending with the highest layer m), their structural equivalence(measured by microscopic isotropy, columnar structure, porosity, anddensity of the layered materials) is maximized. Therefore, their opticalperformance under equal ambient conditions is also matched, as requiredfor high-performance narrow-band filters. To further maximize thestructural similarity and functional equivalence of the matchingelements 30,40, both elements are preferably fabricatedcontemporaneously in the same processing cycle.

In a second stage of manufacture, a filter 50 (shown in FIG. 5) isobtained according to the invention by assembling the two matchingelements 30 and 40 of FIGS. 3 and 4. The two terminating surfaces 38,48are brought into optical contact along a bonding plane 52, therebyensuring that the two half-stacks 32,42 produce a symmetric structure.This assembly step is preferably carried out utilizing the well knownprocess of wet optical bonding, during which the terminating surfaces38,48 are simply wetted and brought into optical contact. This bondingprocess produces an interface at the plane 52 of optical quality atleast as good as is obtained between the film layers deposited duringfabrication.

Over a period of several hours, a strong molecular bond is formedbetween the two surfaces 38 and 48. Such short-range “contact” surfaceforces consist of intermolecular forces between the molecules in the twointeracting materials and the molecules of the wetting fluid medium.Contributors to these surface forces are electrostatic interactionsbetween charged groups on the surfaces, ions and dipoles in the wettingfluid, van der Waals forces due to polarizability of molecules andmaterials, Born repulsion between molecules, and other effects. Theseforces bring the terminating surfaces 38,48 closer together with thepassage of time and the strength of the bond along the bonding plane 52grows as well with time and with the gradual evaporation of themolecular layer of wetting fluid placed between the surfaces.

Since surface forces act at very small distances (typically severalAngstroms), the surfaces 38,48 must be extremely smooth for a welldefined contact area to exist. This is provided by an appropriateinitial polishing of the substrates 34,44. If these substrates arepolished to a residual rms roughness on the order of Angstroms (assmooth as possible, depending on the material, but always less than onenanometer for solar filter applications), the residual roughness doesnot increase substantially as a result of propagation as additionallayers are deposited during manufacture of the multi-layer half-stacks32,42. Thus, the resulting terminating surfaces 38,48 are extremelysmooth, also on the order of a few Angstrom rms roughness, as requiredfor a high-quality optical bond. While the one-nanometer roughnesslimitation is critical for solar filter applications (because excessiveroughness of the dielectric boundaries could expand the filter bandwidthbeyond the width of the Doppler-broadened solar-spectrum line ofinterest and thus reduce the contrast of solar imaging), it is notalways so for other applications, such as in filters fortelecommunication applications. In etalon-based filters fabricated foruse in modern telecommunication fiber-optic applications, greaterroughness may be tolerated because the operating bandwidths of suchfilters significantly exceed the sub-Angstrom bandwidths of filtersrequired for solar observation. For example, the bandwidth of standardsingle-cavity etalon filters used for wavelength locking on the C- andL-band ITU grid in telecom is on the order of 0.5 nm and can toleratedrms roughness in the filter on the order of several nanometers.Therefore, the invention should not be limited to any specific roughnessso long as the degree of polishing is sufficient to produce a bond ofthe quality required for the particular application.

As a result of the two processing stages described above, an optimallysymmetric multi-layer filter structure is always obtained. The maximummicro-structural departure from ideal (that is, non-isotropic,non-amorphous, and porous layered materials) of a filter 50 fabricatedaccording to the invention is significantly smaller than that ofconventional filters and, most importantly, it is optimally symmetric.In fact, to the extent that structural deviations from ideal in anysingle layer (which are inherent to the process of thin-film growth) areamplified as the number of layers in the stack increases, they arereproduced substantially equally in each half-stack of the filter.Therefore, filter performance is not materially affected and thestructural superiority of the filter of the invention producessubstantially narrower spectral bands than analogous conventional filterwith the same number of layers, which is the single most importantadvantage provided by the invention.

Another important aspect of the invention lies in the opportunity itaffords to correct manufacturing errors. The need for correction ofspectral positioning of the filter band may be ascertained routinely byappropriate optical testing conducted during the period of opticalbonding. For example, assume that a single-cavity filter having thestructure of the filter 50 of FIG. 5 is designed to operate at 656.0 nmusing the design specification (HL)⁷H 32L H(LH)⁷, with titania (TiO₂)and silica (SiO₂) materials on silica substrates. As those skilled inthe art will readily recognize, the notation used in the formula aboveindicates the type, number and sequence of the layers deposited to formthe thin-film stack of the filter. H indicates a quarter-wave layer ofhigh refractive-index material (TiO₂); L indicates a quarter-wave layerof low refractive-index material (SiO₂); and the coefficient andexponents indicate repetitions of the relevant layers. Assume that aslightly thicker spacer layer is produced corresponding to thespecification (HL)⁷H 32.1L H(LH)⁷, thereby shifting the peak of thefilter band with respect to the design peak wavelength by about 18Angstrom, as by the peaks labeled Design and Actual in FIG. 6. Such ashift would be unacceptably high for any application that utilizes verynarrow single-emission lines of the spectrum (such as in solar astronomyand telecommunication, for example), rendering a conventionallyfabricated filter unusable and wasted.

A filter fabricated using the method of the invention, however, can bedisassembled along the bonding plane 52 for a period of several hoursafter fabrication simply by soaking the filter in the wetting liquidused to bond the terminal surfaces 38,48 of the half-stacks manufacturedas separate components of the filter. Once disassembled, either one ofthe spacer layers 36,46 can be modified by appropriate additionaldeposition of spacer material in the amount necessary to compensate theunwanted spectral shift by accessing the next spectral order of thefilter. After that correction, the step of optical bonding is repeated.This flexibility of the process of the invention significantly reducesthe cost of fabrication of complex ultra-narrow-band filters andrepresents another significant advance in the art.

As illustrated above, the invention provides a simple and precise methodof controlling the spectral properties of thin-film filters, especiallyultra-narrow-band interference filters, and it may be implemented easilyand at a low cost in any optical device using conventional componentsand known deposition processes. As would be clear to one skilled in theart, appropriate changes can be made with similar results. For instance,the substrate layers 34,44 of the two matching elements 30,40 may be thesame or different, as best suited to a particular application. The twohalf-stacks 32,42 may be deposited at the same time or at differenttimes under the same operating conditions. Similarly, the deposition ofthe spacer layer made up by the two layers 36,46 of FIGS. 3 and 4 can beapportioned in any arbitrary way within the scope of the invention. Forexample, the deposition could be carried out on a single half-stack, asillustrated in FIGS. 7-9, or distributed in any other way between thetwo half-stacks, so long as the aggregate thickness of the spacer layersamounts to the required value. Similarly, the spacer layer 54 may bedeposited during fabrication of the two half-stacks 32,42 or added as aseparate component optically bonded to the half-stacks using the samewet bonding process.

It is also understood that multiple symmetric structures 60,62 can becombined to make optical filters, as illustrated in FIG. 10. In such acase, each symmetric structure can be manufactured advantageously usingthe method of the invention and then advantageously combined using wetbonding. Similarly, as shown in FIG. 11 (and also seen in FIGS. 7 and8), the invention can be used to manufacture filters wherein thesymmetry is limited to a particular section 70 of the half-stacks 72,74of thin-film layers fabricated by deposition. For example, in additionto a number of common layers, one half-stack (72) may include otherlayers 76 designed to produce particular effects. In that case bothhalf-stacks can be fabricated and combined according to the invention toproduce a structure with a limited symmetry with respect to a plane 78passing through the additional layers.

It is also clear that the substrate used to deposit multi-layer stacksof thin films is not limited to plates, but it could be any structuresuitable for deposition. To the extent that two structures can then becombined to form a filter, the procedure can be used to incorporatenarrow-band filters in other optical devices. For example, FIG. 12 showsin three-dimensional view a generic optical device 80 fabricatedaccording to the invention wherein a spectrally adjusted (if necessary)thin-film filter 82 is integrated with two generic, appropriatelypolished, sub-components 84,86 (in this case right-angle optical prisms)acting as substrates for the filter half-stacks. As will be clear to oneskilled in the art, the device 80 acts as a chromatic beam splitterwhere incident light L impinging upon the side 90 of the right-angleprism is split by the filter 82 into a transmitted beam T and areflected beam R, with complementary respective spectral propertiesdetermined by the filter.

Finally, it is also clear that the invention could be carried out bydepositing a single half-stack of thin-film material on a largesubstrate and then cutting the stack vertically across the layers toproduce multiple half-stacks. These could then be combined as describedabove to produce the symmetric structures of the invention.

Thus, while the invention has been shown and described in what arebelieved to be the most practical and preferred embodiments, it isrecognized that departures can be made therefrom within the scope of theinvention. For example, the invention has been described mainly withreference to optical filters for solar applications, but one skilled inthe art would readily recognize that it is equally applicable to themanufacture of optical filters for any other application. Therefore, theinvention is not to be limited to the details disclosed herein, but isto be accorded the full scope of the claims so as to embrace any and allequivalent apparatus and methods.

1. A manufacturing process for a narrowband multi-layer optical filtercomprising the following steps: (a) depositing a plurality of thin-filmlayers of optical material on a first support substrate to form a firstmulti-layer stack of said thin-film layers; (b) depositing saidplurality of thin-film layers of optical material on a second supportsubstrate to form a second multi-layer stack of said thin-film layers;and (c) combining said first and second multi-layer stacks to form asymmetrical structure of said thin-film layers; wherein at least one ofsaid depositing steps (a) and (b) includes depositing a layer of spacermaterial over said plurality of thin-film layers; and said combiningstep includes bonding said first and second multi-layer stacks inoptical contact by wetting the first and second multi-layer stacks witha liquid film in the absence of an adhesive material, opticallycombining the stacks through the liquid film, and allowing the liquidfilm to dry.
 2. The process of claim 1, wherein said depositing steps(a) and (b) are carried out contemporaneously.
 3. The process of claim1, wherein said first and second support substrates are made ofdifferent materials.
 4. The process of claim 1, wherein said depositingsteps (a) and (b) are carried out contemporaneously and includedepositing a layer of spacer material over each of said plurality ofthin-film layers.
 5. The process of claim 1, further including the stepsof repeating steps (a)-(c) to form multiple symmetrical structures andoptically combining said symmetrical structures to produce a narrowbandthin-film filter.
 6. The process of claim 1, further including the stepof testing the symmetrical structure so produced and, if the structuredoes not meet predetermined optical specifications, the additional stepsof separating at least one of the multi-layer stacks from the layer ofspacer material, adjusting the layer of spacer material as needed tomeet said predetermined optical specifications, and again wetting thefirst and second multi-layer stacks with a liquid film, opticallycombining the stacks through the liquid film, and allowing the liquidfilm to dry.
 7. A manufacturing process for a narrowband multi-layeroptical filter comprising the following steps: (a) depositing aplurality of thin-film layers of optical material on a support substrateto form a multi-layer stack of said thin-film layers; (b) cutting saidmulti-layer stack to produce a first multi-layer stack and a secondmulti-layer stack of said thin-film layers; and (c) combining said firstand second multi-layer stacks to form a symmetrical structure of saidthin-film layers; wherein said depositing step (a) includes depositing alayer of spacer material over said plurality of thin-film layers, andsaid combining step includes bonding said first and second multi-layerstacks in optical contact by wetting the first and second multi-layerstacks with a liquid film in the absence of an adhesive material,optically combining the stacks through the liquid film, and allowing theliquid film to dry.
 8. The process of claim 7, further including thesteps of repeating steps (a)-(c) to form multiple symmetrical structuresand optically combining said symmetrical structures to produce anarrowband thin-film filter.
 9. The process of claim 7, furtherincluding the step of testing the symmetrical structure so produced and,if the structure does not meet predetermined optical specifications, theadditional steps of separating at least one of the multi-layer stacksfrom the layer of spacer material, adjusting the layer of spacermaterial as needed to meet said predetermined optical specifications,and again wetting the first and second multi-layer stacks with a liquidfilm, optically combining the stacks through the liquid film, andallowing the liquid film to dry.
 10. An optical filter manufacturedaccording to the process of claim
 1. 11. An optical filter manufacturedaccording to the process of claim
 2. 12. An optical filter manufacturedaccording to the process of claim
 3. 13. An optical filter manufacturedaccording to the process of claim
 4. 14. An optical filter manufacturedaccording to the process of claim
 5. 15. An optical filter manufacturedaccording to the process of claim
 6. 16. An optical filter manufacturedaccording to the process of claim
 7. 17. An optical filter manufacturedaccording to the process of claim
 8. 18. An optical filter manufacturedaccording to the process of claim 9.